I was using older version of WordPress on Ubuntu 16.04. Now when I try to upgrade to WordPress 4.9.5, it says

Unpacking the update...

The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php

Installation Failed

An automated WordPress update has failed to complete

And the plugin upgrade showing

Unable to update WordPress installation/plugins: The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions

Then I changed the owner and group owner to www-data user using below command.

sudo chown -R www-data:www-data /home/username/www/wordpress

After running above command I was able to successfully update WordPress with no further issues.

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EDIT:

After running the command, Whenever I open wordpress, all the folders in it are displayed with a lock icon.

Therefore, when i edit files, its keep asking me to enter root password. This issue fixed by running the below command

sudo chmod 777 -R /home/username/www/wordpress